OSO, Washington (Reuters) - Watching the now murky river from his kitchen window, Bill Best speaks in low tones of neighbors buried by a massive mudslide, where damage to salmon and steelhead trout spawning beds only adds to the grief in a rural Washington state community with deep ties to the land.

"The river's extremely important. We just thoroughly love this relatively natural, unspoiled river environment. It's why we are here," the 73-year-old avid fly fisherman said. "Hopefully in 20 to 30 years, it will be back to what it sort of used to be."

With the death toll from the March 22 mudslide at 36 and rising after millions of cubic yards of earth spliced with trees buried a neighborhood near Oso, the ecological damage to one of the nation's most storied steelhead fly fishing waters is already evident.

The roughly 45-mile (70-kilometer) north fork of the Stillaguamish River has been stained a dark gray-green by silt and nearly blocked by a mud dam, and a pumping system meant to aid search efforts is sending water mixed with sewage and chemicals back into the river.

Migrating salmonids, including federally protected Chinook salmon and steelhead trout, along with their eggs in the river's gravel beds and emerged smolt have surely been killed or will be damaged, state Department of Ecology spill responder Dick Walker said.

Soil samples won't be collected until search efforts end, but the amount of toxic chemicals leaking from households and cars crushed by the slide is relatively low compared to the volume of water, Walker said in an interview. The collapsed hillside and debris field from a massive mudslide that struck Oso is pictured near Darrington, Washington April 2, 2014. Jason Redmond/Reuters

Yet silt, which suffocates spawning beds and harms fish gills as well as insect life, is likely to pile up down river in a Puget Sound estuary used in valley agriculture, and any increased water toxicity could possibly also harm shellfish production, scientists and an industry group say.

"It's going to have an effect on both the adults trying to swim upstream and small smolt salmon trying to swim downstream," Walker said, adding a monitoring station down river has already found damage to migrating smolts.

Between 1,200 and 1,600 Chinook return to the river each year, while steelhead number in the hundreds, said Bill Blake, co-chair of the Stillaguamish Watershed Council, adding it is too early to know the extent of damage to fish populations. A view of the Stillaguamish Valley and part of the community of Oso, Washington, April 3, 2014. Max Whittaker/Reuters

CLEAR AS CRYSTAL

On a recent afternoon in Arlington, near the disaster zone, teenagers were hanging out on the river bank and residents jogged and pushed strollers over a bridge where the date of the mudslide was inked on yellow ribbons tied to a fence.

Beneath the bridge, flecks of splintered wood floated in the north fork's darkened waters as it merged with the healthier south fork, a bright teal.

"What struck me at once was the wonderful transparency of the water," Western novelist Zane Gray wrote of one of the river's tributaries where he hooked a steelhead nearly a century ago. "Deer Creek was the most beautiful trout water I had ever seen. Clear as crystal, cold as ice."

In 1949, the north fork, whose fish have languished from the effects of logging, especially in the mid-20th century, became the first fly fishing-only steelhead water in the world, according to Greg Thomas' Flyfisher's Guide to Washington.

Officials beginning to envision flood-control and rehab schemes have until summer before Chinook salmon begin pushing from the salty Puget Sound into the river system to spawn, Blake said.

This fragile lifecycle is jeopardized by the higher priority of searching for victims among a toxic brew of raw sewage, cleaning products, gasoline, and other contaminants among the mud and splintered trees, Blake said.

"We are in crisis mode along with everybody else," said Eric White, chairman of the Stillaguamish Tribe of Indians. "Spiritually, we rely on that river."

The tribe - whose name means "the river people" - welcomes the arrival of migrating fish in a "first salmon" ceremony and has spent decades working to restore and protect salmon runs. It has traditional fishing rights protected by treaty, White said.

"You don't want to be selfish or disrespectful to the lives that are lost so you have to figure out when to talk about this stuff. 'This river is destroyed' and 'our treaty right' - that's second," White said.

The tribe is planning to use federal emergency funds for river rehabilitation, White said.

Blake, who has seen three mudslides near the river - in 1967, 2006 and this latest one, which killed his friend - said pumps or a series of small ditches installed near the site to be eventually replaced by a new wetlands reserve could help regulate water levels.

Meantime, a temporary berm is being constructed to reduce flood impacts, Snohomish County officials say.

"The landscape was changed completely in four minutes," Blake said. "In a hard way, it reconnects people with the potential of nature. What can be beautiful one minute is also life threatening the next."